Inhibition of energy-linked reactions in chloroplasts by polygalacturonate

Abstract

Swelling and resuspension of spinach chloroplasts in 10 m n sodium polygalacturonate (NaPG) inhibits their light-induced proton uptake more than 99%. Complete inhibition occurs only at low osmotic and ionic strengths; addition of 10 m n salts having nonpermeating anions (succinate, benzoate, tricinate) or of 100 m m sugars (sucrose, mannitol) restores up to 40% of the proton uptake. Addition of salts containing permeant anions (Cl −, NO 3 −, SO 4 −) provides greater restoration, often up to 100% of the original value. Associated with the inhibition of proton uptake is a complete loss of photophosphorylation supported by either cyclic or noncyclic election flow, and of the light-induced light-scattering changes. Electron-transport rates are low but are stimulated to some extent by uncouplers. The absorbance change at 515 nm is inhibited about 40%, and the Ca 2+-dependent ATPase activity is unchanged. In the electron microscope chloroplasts swollen in NaPG demonstrated marked vesiculation of the thylakoid membranes. When proton uptake is restored by NaCl, photophosphorylation activities also return. Sodium chloride addition, however, completely fails to restore the original appearance of the thylakoid membranes in the electron microscope, the light-induced light-scattering changes, or the lost fraction of the 515-nm absorbance change.